Several techniques have been used heretofore to distribute hydrophobic compounds (hereinafter, "hydrophobe"), particularly non-polymeric compounds such as color-forming couplers, ultraviolet light absorbing materials, optical brighteners, etc., uniformly throughout layers of gelatin or other hydrophilic binder materials in the manufacture of radiation-sensitive products. One of the simplest of these techniques involves mechanically dispersing the hydrophobe in solid or liquid form in the binder material by passing a blend of the hydrophobe and material several times through a high energy mill. This technique, however, generally produces unsuitable dispersions which are often unstable.
Another technique is described in U.S. Pat. No. 4,203,716 (issued May 20, 1980 to Chen). That technique involves "loading" polymeric latex particles with the hydrophobe using an organic solvent. Generally, "loading" involves (1) dissolving the hydrophobe in a suitable water-miscible organic solvent; (2) mixing the resulting solution with polymeric latex particles; and (3) removing residual solvent as desired, particularly if necessary to drive the "loading" process to completion, or to provide material sufficiently "loaded" with the hydrophobe. The "loaded" latex is then usually dispersed in a hydrophilic binder in preparation for coating.
Depending upon the hydrophobe, sometimes only a limited amount of hydrophobe can be successfully "loaded" into latex particles, and any residual hydrophobe must be removed to prevent deleterious image effects. Often some of the hydrophobe "leaches" out of the latex particles and forms "crystals." Such crystals deleteriously affect image quality (e.g. reduce sharpness) and, when clumped together, reduce layer smoothness which is important for very thin coatings. This leached-out hydrophobe can also wander into adjacent layers, causing additional problems.
U.S. Pat. No. 3,418,127 (issued Dec. 24, 1968 to Millikan) discloses a method of finely dispersing a fluorescent compound in latex particles by mixing the fluors in polymerizable monomers and emulsion polymerizing the monomers having the fluors therein. The resulting latex purportedly can be coated and dried to form a thin film, preferably over the radiation-sensitive layers of a photographic element. Similarly, W. German Patent No. 2,509,342 (published Sept. 11, 1975) teaches the incorporation of optical brighteners into polymeric particles by dissolving the optical brighteners in polymerizable monomers and emulsion polymerizing the monomers. Emulsion polymerization proceeds in micelles formed by water-soluble surfactant. Additional monomer and hydrophobe migrate from monomer droplets through the water phase and into the micelles prior to polymerization. The resulting latex is purportedly mixed with a compatible colloid (e.g. gelatin) and coated either with a photographic emulsion or in a separate layer in a photographic element.
However, attempts to prepare substantially crystal- and agglomeration-free hydrophilic coating compositions according to the teaching of W. German Patent No. 2,509,342 have been unsuccessful. In particular, as illustrated in Example 1 hereinbelow, the polymer particles of a latex having an optical brightener dissolved therein tended to agglomerate during polymerization. This tendency to agglomerate appeared to increase with time and the resulting polymeric mass could not be coated to form a thin film.
Hence, there is a need in the art for a way to provide relatively inexpensive and simply-made hydrophilic compositions containing hydrophobes which can be coated to provide substantially crystal- and agglomeration-free hydrophilic layers on a substrate.